Chapter 3 Chemical Bonds

Note: order changed slightly Will start with ionic bonding and ionic compounds, then move to covalent bonding and molecular compounds, where Lewis dot structures are used Then will do molecular geometry and VSEPR theory and discuss polarity

Key topics for this chapter Octet rule and how it relates to quantum model of the atom Differentiating between ionic and covalent bonds Naming of ionic and molecular compounds Use of Lewis dot structures and VSEPR theory to determine bonding and geometry of molecular compounds Bond polarity Molecule polarity

Major themes Octet rule: valence electrons make chemistry happen, core electrons play small role Metals react with nonmetals to form ionic compounds Nonmetals react with nonmetals to form molecular compounds We learn the rules for writing names and formulas for ionic and molecular compounds We learn about molecular shape and polarity

Octet Rule

Octet rule

Electron Configurations of Cations and Anions Of Representative Elements Na [Ne]3s1 Na+ [Ne] Metal Atoms ___________ electrons so that cation has a noble-gas outer electron configuration. Ca [Ar]4s2 Ca2+ [Ar] Al [Ne]3s23p1 Al3+ [Ne] H 1s1 H- 1s2 or [He] Nonmetal Atoms ____________ electrons so that anion has a noble-gas outer electron configuration. F 1s22s22p5 F- 1s22s22p6 or [Ne] O 1s22s22p4 O2- 1s22s22p6 or [Ne] N 1s22s22p3 N3- 1s22s22p6 or [Ne] 8.2

Cations and Anions Of Representative Elements +1 +2 +3 -3 -2 -1 8.2

Types of Chemical Bonds According to the Lewis Model Ionic Bonding An atom may lose or gain enough electrons to acquire the same electronic structure as its nearest noble gas neighbor (usually 8 valence e-) and become an ion An ionic bond is the result of the force of attraction between a positive ion (cation) and a negative ion (anion) Occurs between metals and nonmetals

Ionic Bonds

Formation of Ionic Bonds Occur when electrons are transferred from a metal to a nonmetal

Ionic Bond in NaCl

Ionic Bond in CaCl2

Formation of Ionic Bonds Each atom attains a stable electronic structure Number of protons/electrons in atoms no longer equal, charged particles result (ions) Metals lose e-, form + ions (cations) Nonmetals gain e-, form – ions (anions)

A Closer Look at Salt

Salt contains a large number of Na+ and Cl- ions

Ions pack together tightly into crystalline solids Strong attractive force between anions and cations holds crystal together

Types of Chemical Bonds According to the Lewis Model Covalent Bonding An atom may share electrons with one or more other atoms to acquire a filled valence shell. A covalent bond is the force of attraction between two atoms that share one or more pairs of electrons. Occurs between nonmetals

Covalent Bonds A sharing of valence electrons between atoms Usually occurs between nonmetallic elements Nonmetals need to gain e- in order to attain a stable e- structure Thus, when they combine, end up sharing valence electrons

Covalent Bond Atom 1 Atom 2 Attractive forces between nucleus of 1 atom and e- from other atom Repulsive forces between nuclei, electron clouds Attractive forces stronger, bond forms

Covalent Bonds Bohr model of covalent bond By Sharing e- hydrogen obtains a completely filled 1st energy level oxygen obtains a completely filled 2nd energy level (stable octet)

Lewis, or electron, Dot Diagrams Show the valence electrons Consist of element symbol – represents nucleus and core electrons – and dots (valence e-)

Elements and Covalent Bonds Most nonmetallic elements do not exist in nature as individual atoms e.g. Hydrogen exists as a diatomic molecule, H2 By overlapping 1s orbitals, each H atom attains a stable e- structure

Other Molecular Elements Diatomic Elements (Br2, I2, N2, Cl2, H2, O2, F2) Hydrogen All group 7A elements Oxygen • Nitrogen BrINCIHOF!!! Memorize it

General Rules for Bond Types Negative complex ions Nonmetal Covalent Bond Ionic Bond Metal 31

What determines bond type? Generalization metal + nonmetal = ionic bond nonmetal + nonmetal = covalent bond Reality Bonding type is on a continuum, from 100% ionic to 100% covalent 100% covalent 100% ionic

Determining Predominant Bond type Electronegativity is a measure of the tendency for atoms of an element to attract electrons in a chemical bond. Originated with American chemist Linus Pauling (1901-1994), a 2x Nobel Prize winner from Stanford U - a relative scale

Electronegativity (EN) Scale ranges from 0.7 to 4.0

Electronegativity (EN) Trends in Periodic table

Highly electronegative elements (N, O, F, Cl) are greedy for electrons Predominant type of bonding (ionic/covalent) determined by differences in electronegativity (EN) between atoms in bond EN > 1.9, predominantly ionic (metal, nonmetal) EN 0.5 to 1.9, polar covalent EN < 0.5, nonpolar covalent Nonmetal, nonmetal

Determining Predominant Bond Type Look at the electronegativity values of atoms involved in bond Calculate electronegativity difference (EN) Draw arrow in direction that e- cloud is pulled (more electronegative atom) Also use lower case Greek symbol for delta, , along with + or - sign

e- rich Cl H d+ d- e- poor Example: Bond between H and Cl Electronegativity values H= 2.1, Cl = 3.0 EN = 3.0-2.1 = 0.9 = polar covalent bond e- cloud pulled towards Cl atom = Polar Covalent Bond H Cl e- rich electron poor region electron rich region Cl H d+ d- 2.1 3.0 e- poor Direction e- cloud being pulled 9.5

Result of differences in electro-negativity: More electronegative element has greater share of e- cloud, it is electron rich Less electronegative element is e- poor Creation of bond dipoles: + and  -

Example of Nonpolar Covalent Bonds Diatomic elements e.g. H2 Bonds between P and H = 2.1-2.1=0

Bond Character Summary Nonpolar covalent bond  electrons shared equally Polar covalent bond  e- not shared equally, more electronegative atom has greater share of e- cloud Ionic bond – e- transferred from one atom to other, creates a + and – ion